I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to code division multiple access (CDMA) cellular radiotelephone systems.
II. Description of the Related Art
A typical prior art radiotelephone system is illustrated in FIG. 1. This system is comprised of multiple cells covering a geographic region. For clarity, this figure shows only cell A and cell B. The cells are coupled to a mobile switching center (MSC) that routes the calls from the cells to the public switched telephone network (PSTN). The MSC also routes the calls from the PSTN to the appropriate cells communicating with the radiotelephone.
The system may have multiple MSCs, each handling a number of cell sites. The MSCs are linked by voice and signaling trunks. The signaling communication lines can be either a direct link or a network of communication links that introduces a substantial delay on the order of seconds.
In this system, three communication links must be established in order for a radiotelephone to communicate with the public switched telephone network (PSTN). A radio link is first established between the radiotelephone (100) and the base station (105). Next, a link (110) between the base station (105) and the mobile switching center (115) is established. Lastly, the MSC (115) forms a link with the PSTN (120).
As the radiotelephone moves from cell A's coverage area to cell B's coverage area, the radio link between the radiotelephone and cell A becomes weak. Since the radiotelephone is moving into cell B's area, the radiotelephone establishes a new communication path to the PSTN to prevent dropping of the call. The three links described above must now be set up through cell B. The process of moving from one cell to another without dropping the call is referred to in the art as a hand-off.
In code division multiple access (CDMA) cellular systems, this hand-off is typically accomplished as a soft hand-off. A soft hand-off enables the radiotelephone to move from communicating with cell A to communicating with cell B without losing contact with cell A.
In CDMA systems, each cell transmits a unique pilot signal that identifies the cell to the radiotelephone. When the radiotelephone is communicating with cell A, it is continuously monitoring pilot A as well as other pilot signals from neighboring cells, such as cell B. CDMA systems in general and pilot signals in particular are described in Telecommunications Industry Association/Electronic Industry Association Interim Standard - 95A (TIA/EIA IS-95A).
As the radiotelephone moves away from the coverage of cell A, the strength of pilot B increases while the strength of pilot A decreases. The radiotelephone continues to monitor pilot A as long as the pilot remains above a predetermined threshold defined in IS-95A.
If the strength of pilot B exceeds a certain threshold, the radiotelephone sends a Pilot Strength Measurement Message to cell A, as described in IS-95A, indicating that the radiotelephone wishes to establish a radio link with cell B. Cell A forwards the message to the MSC so that the MSC knows to allocate another path from the radiotelephone to the PSTN. After the path is allocated, the radiotelephone is directed to communicate with cell B.
The radiotelephone, however, does not stop communicating with cell A unless pilot A has dropped below the minimum strength threshold. While the radiotelephone is communicating with both cells A and B, the cells forward the signal received from the radiotelephone to the same MSC. The MSC picks the best received signal and transforms the signal to a PSTN compatible signal in the pulse code modulated (PCM) format and directs the PCM signal to the PSTN. Signals received by the MSC from the PSTN are handled in a similar, reverse manner.
When the radiotelephone has moved away from cell A so that pilot A has dropped below the minimum threshold, the radiotelephone breaks communication with cell A. The radiotelephone, however, remains in contact with the PSTN through cell B. This is referred to in the art as an intra-MSC soft hand-off since the same MSC is used by both cells.
A problem exists when cell A uses one MSC and cell B uses a different MSC. In this case, due to the time delay introduced by the signaling communication network between the MSCs, the inter-MSC soft hand-off is prevented from occurring reliably and, therefore, increases the chance that the call will be dropped when the radiotelephone changes from one cellular system containing the initial MSC to another cellular system containing the other MSC. There is a resulting unforeseen need for a system that performs a reliable, inter-MSC soft hand-off.